Quick fixes from the first boot: recovery admin, upsell, warnings

- The recovery administrator (INBUXA_RECOVERY_ADMIN, or STALWART_RECOVERY_ADMIN)
  is honored only in bootstrap and recovery mode. On a configured server it's
  ignored with a startup warning. Before, it was a standing full-admin login
  for as long as the variable stayed set.
- The Enterprise upsell error is replaced by "This feature isn't available in
  INBUXA yet" for the features still to be rebuilt.
- Workspace warnings: 25 to 0. cargo fix removed the unused imports. The
  seven places where Enterprise code used to plug in keep their parameters,
  each with an inbuxa: comment naming the rebuild that uses it again. The
  antispam test's mock-server imports are back behind pending-rebuild.

crates/common/src/lib.rs

@@ -161,6 +161,9 @@ pub struct Data {
 pub struct LogoCache {
     domain_id: u32,
     tenant_id: Option<u32>,
+    // inbuxa: read again when the /logo endpoint (per-tenant and per-domain
+    // branding) is rebuilt; docs/spec/features/multi-tenancy.md MT-22.
+    #[allow(dead_code)]
     data: Option<Resource<Vec<u8>>>,
 }